1. Field of the Invention
The present invention relates to a process for the bonding of metallic materials and for the production of coatings on metallic materials with a polyvinyl chloride plastisol having improved adhesion and paintability at baking temperatures of 90.degree. C. and up, which plastisol contains, as an adhesion- and paintability-improving additive, a condensation product of a polymerized fatty acid, and a polyalkylene polyamine together with an N-aminoalkylpiperazine.
Polyvinyl chloride and its copolymers have long been known to possess resistance to deterioration caused by the action of aggressive media and they are, therefore, widely used to impart corrosion resistance to metallic surfaces, to bond thin sheet metal structures and to seal welds, especially on preprimed metal in the automotive field.
Such coatings are applied to the surface of the material to be protected mainly in the form of a plasticized vinyl chloride polymer (plastisol) by spread coating, roller coating, spray coating or the like. In one widely used form, a plastisol coating material is prepared by combining a plasticizer or mixture of plasticizers, a polyvinyl chloride, formable into a paste and distinguished in particular by a definite swelling capacity in the plasticizer, one or more fillers, stabilizers and, optionally, pigments and PVC processing aids.
2. Description of the Prior Art
The formulation of plasticized polyvinyl chloride (PVC) coating materials, their preparation and methods of applying them are described in great detail in Krekeler Wick, Kunststoff-Handbuch ("Plastics Handbook"), 1963, Vol. II, Part 1, pp. 396 et. seq.
The adhesion of the plastisol to the material to which it is applied is an important criterion for its performance. This is particularly true when the plastisol is used as a coating on preprimed metal parts. Poor adhesion of this PVC protective coating to either bare or preprimed metal reduces the efficiency of the protective coating. It also increases the likelihood of penetration by aggressive media. Water, for example, can readily seep in between the coating and the primed metal and degrade the integrity of the system. The poorer the adhesion of the protective film to the metal, the greater the likelihood that this will occur. An additional advantage of the plastisol of this invention is the inherent flexibility of the cured plastisol when compared to competitive systems such as acrylics.
The paintability of plastisols applied to materials such as metallic surfaces at low-bake temperatures is important in the automotive industry which applies top coatings to plastisols that are cured by acid catalysis at low-bake temperatures. The basicity of the adhesion promoter should not, therefore, interfere with such paint systems
Thus, there has been a need for plastisols which make it possible to produce high-strength bonds between a wide variety of materials, especially metallic materials, and to apply acid-cured top coatings to these materials especially at low-bake temperatures. According to U.S. Pat. No. 4,146,520, polyaminoamides having a certain imidazoline content and made from a polymerized mixture of fatty acids with a high content of trimeric and higher polymeric fatty acids and an excess of polyalkylene polyamines, are used as adhesion promoters in an amount from 0.5 to 5 weight percent, based on the plastisol mass.
According to U.S. Pat. No. 4,717,746, heterocyclic amines of the formula ##STR1## wherein R is hydrogen or an aminoalkyl group, --R'--NH.sub.2, in which R' is a saturated aliphatic chain, form adhesion promoters when condensed with polymerized fatty acids. However, when such promoters were used at relatively low cure temperatures, for example below 140.degree. C., the adhesive properties of the plastisols were found to be below acceptable commercial levels as described in Table III.
These polyaminoamides effectuate a definite improvement in adhesive strength. When used in low concentrations, however, and at low baking temperatures, they leave room for further improvement with respect to adhesion, thermal stability, elongation at rupture and paintability of the cured PVC plastisol.